There has conventionally existed a technology in solid-state image sensing device whereby phase-difference focus detection is performed by dividing a photodiode on which light is concentrated by one micro lens that is in a single pixel. In Japanese Patent Laid-Open No. 2001-083407, the photodiode in one pixel is divided into two, allowing the photodiodes to receive light from different pupil faces of an image capturing lens. Focus detection is performed with the image capturing lens by comparing the output from the two photodiodes. An approach also exists whereby the output from such a solid state image sensing device is used not only for focus detection, but also as an image signal.
In Japanese Patent Laid-Open No. 2001-083407 mentioned above, focus detection is performed by reading signals of two photodiodes separately from the image sensing device and calculating a phase difference. Furthermore, when the signals from the photodiodes are added together, they can be used as an image signal. The more data there is, the more accurately focus detection can be performed, and therefore it is preferable to read signals from photodiodes in all pixels in an image sensing device individually and detect phase differences, after which the signals of all the photodiodes are added together to produce an image signal. However, it takes time to read the signals from the photodiodes in all the pixels individually. For example, if two photodiodes are provided in the horizontal direction in an image sensing device that has 4000 pixels in the horizontal direction, there are 8000 photodiodes in the horizontal direction, which results in a dramatically longer reading time than in an ordinary image sensing device in which the photodiodes are not divided.